Simulcast systems utilize multiple transmitter sites to transmit a signal of interest, where the multiple transmitter sites operate on the same carrier frequency, to generate overlapping coverage areas. The multiple transmitter sites achieve a greater geographic coverage area than can be achieved by using a single transmitter. Typically, multiple simulcast transmitters transmit the same signal, on the same frequency/channel, and at the same time, so that receivers within the overlapping coverage areas receive the transmitted signal from the multiple transmitters simultaneously, without interference.
Interference typically occurs when multiple transmitted signals arrive at a receiver at slightly different times, out of phase with each other. This is often due to differences in propagation delays, multipath distortion, and other factors. When a receiver receives signals from two or more transmitters at slightly different times, a form of distortion known as Simulcast Delay Spread (SDS) distortion is created. Under certain conditions this distortion may become severe, and corrupt the received signal to an unacceptable degree.
The interference can be eliminated by compensating for the propagation delays in the simulcast system, for example, by controlling the relative timing of the respective signal transmissions, so that the propagation delays are offset and thereby eliminated. For example, the interference among the multiple transmitted signals can be minimized by synchronizing the timing of each signal transmission, so that each signal is transmitted at the same time, and at the same frequency, as all the other signals in the simulcast system. Controlling the precise timing is critical, and oftentimes problematic, because even small timing variances can significantly distort the received signal.
Several systems and methods have been introduced in the prior art to provide precise synchronized transmit timing. U.S. Pat. No. 6,011,977 ('977 patent) teaches a method and apparatus relating to simulcasting, in which a timing pulse train and data signal are received at a transmitter site. The pulse timing characteristics of the received timing pulse train are compared with the pulse timing characteristics of a locally generated timing pulse train, to determine an appropriate delay to be used to effect synchronous transmission of the received data signals at their respective transmission sites.
An existing system, the SynchroCast systems®, manufactured by Harris Corporation, includes a studio site and multiple transmitter sites. At the studio site, two reference signals REF1 and REF2 are generated, basically 9600 Hz clocks with three types of embedded flags, referenced from a local Global Positioning System (GPS) satellite receiver. Those reference signals are sampled and encoded for transmission to transmitter sites. The sampled and encoded signals are transmitted to transmitter sites over a TDM network such at T1 or E1.
At the transmitter site, 1 PPS and 10 MHz signals are obtained from a local GPS receiver and are synthesized into delayed 1 PPS and 9600 Hz signals. The signals REF1 and REF2, received from the studio site, are decoded at the transmitter sites to produce the embedded flags, which are compared to the versions of the same produced by the local GPS receiver. From discrepancies, appropriate delays are determined, which are used for nulling out delays between the embedded flags and local time markers. Thus, the existing Harris SynchroCast system transmits time information by embedding time markers into 9600 Hz pulse train signals which are transmitted across a T1/E1 network connection.
A disadvantage of the pulse train approach is that the rate in which time comparisons can be made at the transmitter sites is limited to once per second with most GPS receivers. A custom GPS receiver can be obtained to increase the frequency of its output to greater than 1 PPS, but unfortunately the size of the delay measurement window decreases as well. For example, with a 1 PPS GPS signal, delays can be measured up to a one second maximum.
Accordingly, there is a need for a more responsive system and method to effect synchronous transmission of signals in a simulcast system. Particularly, a system that provides greater precision through more frequent timing measurements, more flexible configurations and a reduction in operating parts, and has a reduced product cost than the systems currently in use.
Another disadvantage of the existing prior art simulcast system is that they do not work over packet switched network links like UDP/IP.